The present invention concerns improvements in apparatus which treats pure steam, or steam prepared by evaporation of purified water, and provides the apparatus in which phenomenon of discoloration, which is called "fouling", on the surface of the apparatus which may occur after use over a long period of time.
Needless to say, water system is an important part of various industries such as biotechnology and pharmaceuticals. For example, water for injection and purified water must observe regulations by pharmaceutical codes, and sterilization by high pressure steam is carried out using pure steam. Pure water and pure steam are used also in some fields other than the above mentioned industries, such as food industry, fermentation and brewing, and cosmetics.
Pure steam is generated by distilling purified water, which in turn is prepared by purification methods such as deionization, reverse osmosis and ultrafiltration, and the pure steam-related apparatus such as pure steam generators, pipelines for passing the pure steam, sterilizer using the pure steam and condenser for obtaining condensed pure water are made with stainless steel, particularly, austenitic stainless steel which is of good processability. The material called "sanitary stainless" is used for manufacturing the pure steam-related apparatus in the state that it has been surface polished so as to eliminate unevenness which may be strongpoints for microorganisms to propagate. Polishing is usually carried out by combination of belt polishing using a fine abrasive grain of #180 or finer and subsequent buff polishing so that highly smooth surface may be obtained.
Pure steam is so corrosive that those in this technology often observe formation of "rouge" which is, as the name indicates, red metal oxide on the surfaces of the sanitary stainless products which are in contact with pure steam. This is reasonable in view of the fact that purified water has, as is called "hungry water", aggressive nature to produce corrosion on the surface of the metal. Formation of rouge must be prevented because the rouge may go into condensed water to damage water quality or contaminate the objects to be sterilized.
To cope with formation of the rouge it has been proposed to use the sanitary stainless treated by, following to the above mentioned mechanical polishing, electropolishing, or, instead of, or together with the electropolishing, passivating the surface of the apparatus (J. Villafranca et al., Pharm, Engineer., 5(6), 28-30, 1985). However, even with the electropolishing and passivation, it is still difficult to prevent occurrence of the rouge. Further, it was observed that discoloration to black occurs on the surface of the sanitary stainless. The black discoloration tends to occur at the parts where pure water formed by condensation of pure steam exists. These phenomena of staining the surfaces of sanitary stainless steel parts are in all called "fouling".
There are various views on the relation between the methods of surface treatment of austenitic stainless steel and corrosion thereof. The above noted proposal to electrolytically polish the surface is based on the report that corrosion resistance of the steel to pure hot water and pure steam will be improved when electropolishing is added after buff polishing in comparison with buff polishing only (K. Nagai, PHARM TECH JAPAN 11(13), 25, 1995). On the other hand, another researcher reported that Cr-content at the surface, which is believed to be an index of corrosion resistance, is higher in the passivated surface than in the electrolytically polished surface (M. Seo et al., Trans. Jpn. Inst. Met., 21(12), 805, 1980). Further, according to another report, oxidation treatment in gas phase and acid treatment in addition to the electropolishing decreases dissolution of the stainless by hot water, acid or alkaline solution (T. Tasaki et al., Kuki Seijo, 31(4), 232, 1993). Anyway, however, there has been established no effective method to prevent fouling or discoloration to red or black of the sanitary stainless surface.
As the temporary countermeasure it has been recommended and practiced, particularly in the United States, to repeat passivation treatment periodically after installation and operation of the apparatus related to pure steam. In Japan, such a treatment has not been established and seldom practiced. If the passivation after installation of the apparatus is to be practiced, it is necessary to introduce solution for passivation by pumping from a storage tank to process lines of the apparatus, to keep the apparatus under contact with the solution for a necessary period of time and to rinse the apparatus with water. When necessary, pumped circulation or heating the passivation solution in a jacketed storage tank will be added to the above operation. Anyway, re-passivation is a laborious operation. The spent passivation solution must be treated before discarding, and such treatment will require considerable cost.